A silica glass is used for a lens, a prism and a photomask of a photolithography instrument in manufacturing of a large-scale integrated circuit (LSI), for a TFT substrate used for a display, for a tube of a ultraviolet lamp or an infrared lamp, for a window material, for a reflection plate, for a cleaning container in a semiconductor industry, for a container for melting of a silicon semiconductor, and so forth. However, an expensive compound such as silicon tetrachloride must be used as a raw material for a silica glass; on top of that, melting temperature and processing temperature of a silica glass is extraordinary high (about 2000° C.), thereby leading to a high energy consumption, and this in turn to causing enormous emission of carbon dioxide, which is considered to be one of the global warming gases. Accordingly, from the past, a method for producing a silica glass by using a relatively cheap raw material has been sought.
For example, in Patent Document 1, a method (sol-gel method) in which a silicon alkoxide is hydrolyzed to a silica sol, which is then gelated to a wet gel, then to a dry gel by drying, and finally to a transparent silica glass body by firing at high temperature is disclosed. In Patent Document 2, a method in which a transparent silica glass is obtained by a sol-gel method from a silica sol mixture solution formed of tetramethoxy silane or tetraethoxy silane and a silica sol solution containing silica fine particles is disclosed. In Patent Document 3, a method for producing a transparent silica glass by using a silicon alkoxide and silica glass fine particles as its main raw materials, wherein a heating process at a temperature range of 200 to 1300° C. is carried out under an oxygen gas-containing atmosphere, a further heating process to 1700° C. or higher is carried out under a hydrogen gas-containing atmosphere, and a heating process between the foregoing two heating processes is carried out under a reduced pressure atmosphere, is disclosed. In these conventional sol-gel methods, however, the produced silica glass has problems not only in an initial dimensional precision of the produced silica glass and in a heat resistance during its subsequent use at high temperature but also in its production cost which is not so cheap.
In Patent Document 4, a method (slip casting method), wherein at least two different kinds of silica glass particles, for example, silica glass fine particles and silica glass granules are mixed to obtain a water-containing suspension solution, which is then press molded and sintered at high temperature to obtain a silica-containing composite body, is disclosed. In Patent Document 5, a method, wherein a mixed solution (slurry) containing silica glass particles having the size of 100 μm or less and silica glass granules having the size of 100 μm or more is prepared, then the slurry is cast into a molding frame, dried, and then sintered to obtain an opaque silica glass composite material, is disclosed. In these conventional slip casting methods, however, shrinkage of a molded article in a drying process and a sintering process is so large that a thick silica glass article with a high dimensional precision could not be obtained.
Accordingly, there are problems as mentioned above in the method for producing a silica glass article from a powdered raw material. Therefore, as a method for producing a silica crucible for manufacturing of a single crystal silicon used for LSI, such production methods as those disclosed in Patent Document 6 and Patent Document 7 are being used still today. In these methods, after a powdered ultra-highly purified natural quartz or a powdered synthetic cristobalite is fed into a rotating frame and then molded, carbon electrodes are inserted from the top and then electrically charged to cause arc discharge thereby raising the atmospheric temperature to a temperature range for melting of the powdered quartz (temperature is estimated in the range from about 1800 to about 2100° C.) so that the powdered raw quartz may be melted and sintered.
In these methods, however, there has been a problem of a high cost because powdered raw material quartz with high purity is used. In addition, because various kinds of impure gases are dissolved in a produced silica crucible, the gases are released and then incorporated into a single crystal silicon as gaseous bubbles when it is used as a silica crucible for growing of a single crystal silicon; this and others have been causing problems in production cost as well as quality of the silicon crystal. In addition, there has been a problem about a thermal distortion resistance of the silica crucible because side wall of the crucible is distorted by softening at the time of pulling up of a single crystal silicon.
In Patent Document 8, a silica crucible, obtained from a powdered silica raw material by an arc-discharge melting method and formed of three layers of an outer layer comprised of a natural quartz glass, an intermediate layer comprised of a synthetic quartz glass containing aluminum in high concentration, and an inner layer comprised of a high purity synthetic quartz glass, is described (it seems that the melting was carried out under an air atmosphere). In it, the prevention effect of impurity migration by the intermediate layer is shown. However, not only a high cost of the three-layer structure having the structure as mentioned above but also the problems of thermal distortion resistance have been remained unsolved.
In Patent Document 9, a technology to reduce gaseous bubbles in a wall of a melted quartz crucible by aspiration from a peripheral of a molding frame at the time of an arc-discharge melting of a molded article of a powdered raw material silica is shown.
However, dissolved gases in a wall of a melted quartz crucible could not be removed completely by mere aspiration of an air present in the powdered silica. Particularly, it was only possible to produce a crucible containing a large amount of residual H2O gas.
In Patent Document 10, a silica crucible, formed of three layers containing a crystallization-facilitating agent and produced by a similar arc-discharge melting method, is shown.
However, when a single crystal silicon is pulled up by using this three-layered crucible, there have been problems that the crucible is not necessarily crystallized uniformly, defects such as voids and pinholes are formed in a grown single crystal silicon because of a large amount of released gases from the crucible, and the thermal distortion takes place at the time of using the crucible.